CHAPTER 8 GENERATOR TESTING

8.1       GENERAL

The general requirements for the testing of all rotating electrical machines, including generators, are laid down in BS 4999 : 1976, Part 60.

As far as generators are concerned, the principal requirements are described below.

8.2       MANUFACTURER’S TESTS

Manufacturer’s tests are given three classifications: ‘Basic’ (formerly called ‘Type Tests’), ‘Duplicate’ and ‘Routine Checks’.  Basic tests are mainly to prove a new design.  They include exhaustive tests to ensure that the design meets the specification and all other performance requirements.  They are normally carried out only on the ‘first of class’ generator, and a Test Certificate is provided on request to confirm the tests.  Basic tests may, on special request, be repeated on the first machine of a new, large order, but this is not usual.

Duplicate tests are for performance.  They are applied to a generator that is of the same design and construction as one previously made (and in no way altered) and which has already undergone basic tests.  The duplicate tests are to ensure that the generator is still in accordance with the original design.

Routine checks are tests to show that each individual generator has been assembled correctly, is able to withstand the appropriate high-voltage tests and is in sound working order both electrically and mechanically.

The three classes of test are listed in Table 1.

TABLE 1 - MANUFACTURER’S TESTS

Test	Basic	Duplicate	Routine
Resistance of windings (cold)	X	X	-
No-load losses	X	X	X
Temperature rise	X	-	-
Tests for efficiency	X	-	-
Momentary overload	X	-	-
High voltage	X	X	X
Vibration	X	-	-
Short-circuit saturation	X	X	-
Short-circuit losses	X	-	-

Most of these tests are self-evident, but the following additional information is given on the high-voltage test.

For this test, also called a ‘withstand’ test, a high voltage is applied between the frame and all the generator stator win dings, with all other conductors, metal and auxiliary (i.e. heater) circuits bonded to the frame.  The actual voltage applied is in accordance with Table 2 and is sustained for one minute.  It may be at any frequency between 25Hz and 100Hz.  It is primarily an insulation test for the generator’s windings and is included also in the routine checks to ensure that there has been no fault during assembly of any individual machine.

TABLE 2 - HIGH VOLTAGE TESTS

Windings	Test Voltage (rms)
Generator stator windings: <100v br="" kva=""> >100V, <1kva br=""> 1 – 10 000kVA >10 000kVA and <2 br="" nbsp=""> 2 000 - 6 000V 6 000 – 17 000V >17 000V	500V + twice rated voltage     1 000V + twice rated voltage     1 000V + twice rated voltage, min 500V     1 000V + twice rated voltage     2.5 times rated voltage     3 000V + twice rated voltage     Special agreement
Rotor windings (including related exciters):	10 times the excitation voltage         minimum 1 500V         maximum 3 500V

8.3       ON-SITE TESTS

Any generator installed on a platform or in an onshore oil installation may be assumed to have undergone its full routine check tests, and its prototype a full basic or duplicate test.  On-site tests are therefore only needed to check the original installation and commissioning, and thereafter to ensure that no deterioration has taken place.  The remainder of this chapter deals only with tests for the latter purpose.

Deterioration can occur for many reasons: among them are entry of dampness or water leakage in the generator or cable-entry boxes, overheating of the windings due to overloading, or mechanical faults such as vibration or bearing failure.

Both dampness and overheated windings can cause reduced insulation resistance of the windings.  After drying out, the generator should be megger tested to ensure that insulation resistance has been restored.  Deterioration can be progressive, especially when a machine is little used, and a regular programme of megger testing every generator should be drawn up and the results logged.  After temperature correction (see para. 8.4), the resistance levels should be plotted, and, if there is progressive deterioration, this will be immediately apparent.

After repairs to a generator, a megger test should normally be carried out before reconnection if the generator or its connections have in any way been interfered with.

High-voltage ‘withstand’ tests should never be needed on site unless a major overhaul has been carried out, in which case it would be an engineering or manufacturer’s concern.

8.4       MEGGER TESTING

‘Megger’ instruments are provided which operate at 250V, 500V or 2 500V, and the correct one must be used depending on the rated voltage of the generator to be tested.  Normally generators over 415V and all high-voltage generators would require a 2 500V megger.

When the megger is connected and the handle wound up, the voltage should continue to be applied until the needle settles down to a steady value; this might take one minute or more.

When testing the insulation resistance of a winding, all other conductors, metalwork (stator and rotor) and auxiliary circuits such as those for thermistor protection and heaters should be connected to the frame with light wire (fuse wire will do), and the test voltage applied between winding and frame.  This is to ensure that not only is the insulation to earth satisfactory for the winding under test, but also that it is adequate to other circuits and elements which are not normally at earth potential - e.g. heater elements and circuits.  Where the 3-phase windings are independent and brought out to six terminals, it is advisable to make a test also between pairs of windings by removing the star-point links.  However, where generators are star-connected with their star-point internal and permanently made, inter-phase tests are not possible.

Insulation resistance is very dependent on temperature, and, in order to compare one reading with another, it is necessary to reduce the value to a common temperature.  This is usually 40^oC.  Unlike the resistance of a conductor, which rises with temperature, the resistance of insulation falls rapidly with increase of temperature.

The graph of Figure 8.1 is used to make this correction by means of a ‘temperature coefficient’.

For example, if the observed reading (R_t) is 10 megohms when taken at 70^oC, then, using the graph, the temperature coefficient (K_t) is 8.0, and the corrected reading at 40^oC (R₄₀) is then 10 x 8.0 = 80 megohms.  It can be seen from this example that the correction is considerable when the winding is hot at normal working temperatures.

FIGURE 8.1

INSULATION RESISTANCE TEMPERATURE COEFFICIENT

The recommended minimum value of insulation resistance for generators is given in manufacturers’ literature.  As a guide when precise information is not available, the minimum acceptable value (R_m) for a generator stator winding is given by:

                                R_m = (kV + 1) megohms when corrected to 40^oC,

where kV is the generator’s rated voltage in kilovolts.  Thus:

for 415V or 440V generators	Rm ‡ 1.4MΩ
for 6.6kV generators	Rm ‡ 7.6MΩ
for 11kV generators	Rm ‡12MΩ

CHAPTER 7 GENERATOR PROTECTION

                                                             

7.1       GENERAL

Electrical plant can be damaged, or destroyed, by operation outside its designed ratings or by fault conditions caused by a breakdown of some part of the system.

The automatic protection of electrical installations, including generators, against such damage is described fully in the manual ‘Electrical Protection’.

In brief, generators are protected against some or all of the following abnormal conditions:

·                     Overcurrent

·                     Earth fault

·                     Differential current

·                     Reverse power

·                     Overvoltage

·                     Undervoltage

·                     Underfrequency

·                     Field failure

·                     Diode failure

·                     Winding overtemperature

·                     Turbine trip

Overspeed.

CHAPTER 6 DIESEL GENERATOR SETS

6.1       GENERAL

In large onshore installations power is derived from the National Grid.  On platforms the main generating sets are always driven by gas turbine, using the platform’s own gas as fuel when available, with liquid fuel as an alternative in some cases.

Onshore the grid supply can sometimes fail, and on platforms main generators may also fail, or under certain conditions they may be deliberately shut down.  In either case there is loss of main power supply, and it is important that there should be immediately available a quick-starting alternative supply - and this means diesel generation.

All platforms, and most large onshore installations, have one or more diesel-generator sets.  In many cases they are arranged to start automatically on loss of mains voltage and to switch themselves onto an emergency switchboard.  It is never the intention that such generators should replace the lost main ones, but they should provide limited power for only really essential services such as some degree of lighting, safety, instrumentation, communications, fire and gas detection and so on.

Diesel-driven generators are also required for ‘black-start’ conditions when no main generators are running but whose auxiliaries must be run in order to start them.  Such diesel sets must of course be entirely self-contained, requiring no external assistance to start them.

The construction of a diesel engine is well known and will not be described here.  It is usually multi-cylinder, turbo-charged and jacket-cooled through a water/air radiator, some times assisted by a cooling fan.  It is usually battery-started, and some sets have an alternative hydraulic starter, hand pumped, for use if the battery becomes discharged, for example after a prolonged shutdown.  It is vitally important for diesels which drive emergency generators which are automatically started that the batteries are maintained fully charged ready for an instant start; also that practice starts should be exercised regularly.

6.2       BASIC SERVICES

In all installations the really essential services, which it is vital to keep running even when the normal main power has been lost, are offshore termed Basic Services.  The diesel-driven generator is called the ‘Basic Services Generator’ and its switchboard the ‘Basic Services Switchboard’, both shown in red in Figure 6.1.  The system is usually at low voltage (440V), and positive steps are taken to see that the basic services generator does not feed back into any non-basic low-voltage services or into the high-voltage system.  (There are however some exceptions to this practice.)

Under normal conditions on an offshore platform the basic services switchboard is part of the complete 440V distribution system.  It is in continuous use and is normally fed through an interconnector from a main 440V board, as shown in Figure 6.1.  If power on the main board fails, the basic services board is isolated from it and can be fed direct by its basic services generator, which normally has sufficient capacity for that board and no more.  The generator may start automatically on failure of the main 440V power, but quite commonly it must be manually started.  The incomer circuit-breaker from the generator is interlocked with the incomer from the main 440V board so that both cannot be closed at the same time; therefore the generator can never feed back into the remainder of the 440V system or, through the transformer, into the HV system (other than with the exceptions mentioned above).

FIGURE 6.1

TYPICAL BASIC SERVICES AND BLACK START GENERATOR ARRANGEMENT

Auto-starting is achieved by providing the basic services busbar with an undervoltage relay which causes the interconnector to open on loss of main supply and the basic services generator to start.  When the generator has started and run up, it closes its incomer breaker automatically and in so doing locks out the interconnector.  Even when main power is restored, the interconnector breaker cannot be reclosed onto the basic services board until the operator has first opened the generator incomer breaker, so lifting the interlock.  The normal interconnector incomer breaker can then be closed, and the system reverts to normal.  The basic services generator is afterwards stopped manually and left in a condition to restart whenever needed.

Where the start is manual no undervoltage trip is provided, but instead the act of manually closing the generator incomer breaker also trips and locks out the interconnector incomer breaker.  When power is restored, the process is reversed manually.

With regard to the exceptions referred to above, on some of the newer platforms larger diesel-generator sets are fitted which have a capacity appreciably greater than that needed only for the basic services switchboard and its essential loads.  In those cases some limited feedback into the system is allowed to power other less essential but still important loads, such as utilities.  In that case the interlock between the generator and interconnector breaker is not fitted.

6.3       AVAILABILITY OF BASIC SERVICES GENERATOR

A basic services generator is nearly always needed in a hurry, whether automatically or manually started.  It is therefore always left in a ‘ready-to-run’ state.  If automatic, the selector switch is left on ‘Auto’, even if it had been turned to ‘Local’ for the previous manual stopping.  Ready-use fuel tanks are kept full, oil and water levels correct, battery fully charged and heaters on.  These things are checked daily, and always after the machine has been run.

Where basic services generating sets are automatically started on loss of main supply, this feature is regularly tested to ensure that it functions correctly.  Manual starts on all auxiliary sets are also regularly exercised.

6.4       BASIC SERVICES GENERATOR UTILITIES

Most diesel engines are electrically started from a local battery, usually 24V.  When the engine is not in use this battery is kept fully charged by a charger fed from the main a.c. system.  An engine-driven d.c. generator charges the battery when the engine is running.

Basic services diesel engines are provided with ready-use fuel tanks with a capacity sufficient for at least 24 hours’ full-load running.  As main supplies are assumed to have been lost, fuel pumping facilities may not be available, and it may be necessary to refill the tank by hand-pumping from barrels.

Each diesel generator unit is provided with a local control panel on or near the engine mounting, from which the output can be controlled and monitored for speed and voltage.  No remote control is exercised from the Electrical Control Room on the generator and interconnector circuit-breakers.  All control is local, but there is usually some remote instrumentation in the Electrical Control Room.